Download - TITLE : SALT Instrument Calibration System · TITLE : SALT Instrument Calibration System ... lamps; calibration screen; illumination PREPARED BY : David Buckley SALT Project Scientist

Calibration Specifications

Doc No. SALT ???????? Issue A

1

APPROVAL SHEET

TITLE : SALT Instrument Calibration System

DOCUMENT NUMBER : 15??AS0000 ISSUE: A

SYNOPSIS : This document describes the technical requirements for theSALT calibration systems, comprising the Payload InstrumentCalibration System and the Dome Instrument CalibrationSystem. It defines the functional, operational, software andhardware requirements that must be met.

KEYWORDS : Science instrument calibrations; payload instrumentcalibration system; dome instrument system; calibrationlamps; calibration screen; illumination

PREPARED BY : David BuckleySALT Project Scientist

APPROVED :Leon NelTCS PROJECT MANAGER

Gerhard SwartSALT SYSTEMS ENGINEER

DATE : 26 November 2002



2

This issue is only valid when the above signatures are present.



3

ACRONYMS AND ABBREVIATIONS

CCAS Centre-of-Curvature Alignment SensorCCD Charge-coupled Device (Camera)CFE Client-furnished EquipmentCOTS Commercial off the shelfDICS Dome Instrument Calibration SystemDP Data ProcessorDec DeclinationEE(50) Image diameter containing 50% of Enclosed EnergyEL Event Logger (computer)FoV Field-of-ViewGUI Graphical User InterfaceHET Hobby-Eberly TelescopeHRS High-resolution SpectrographI/O Input/Output (Device)ICD Interface Control DossierIR InfraredLRS Low-resolution SpectrographMMI Man-Machine InterfaceMPB Moving Pupil BaffleMTBF Mean Time Between FailuresMTTR Mean Time to RepairMOS Multiple Object SpectroscopyOCS Observatory Control SystemPC Personal ComputerPICS Payload Instrument Calibration SystemPFIS Prime Focus Imaging SpectrographPFP Prime Focus PayloadPI Principal Investigator (Astronomer)PM Primary MirrorRA Right AscensionSA SALT AstronomerSAAO South African Astronomical ObservatorySAC Spherical Aberration CorrectorSALT Southern African Large TelescopeSICS SALT Instrument Calibration SystemsSNR Signal to Noise ratioSO SALT OperatorSW SoftwareTAC Time Assignment CommitteeTBC To Be ConfirmedTBD To Be DeterminedTCS Telescope Control SystemUPS Uninterruptible Power Supply



4

DEFINITIONS

Acquisition time This is the length of time required to put the target at adesired position (a bore-sight), within the offset pointingrequirement, from end-of-slew, until start of theintegration.

Arc A type of lamp (e.g. hollow cathode arc lamp) used tocreate an emission line spectrum which is used towavelength calibrate spectroscopic data.

Baffle A mechanical aperture or stop used to exclude unwantedlight rays from entering an optical system.

Calibration A procedure employed by the SA and/or PI to remove anyinstrumental effects (e.g. introduced by the telescopesystem or the science instrument) from an astronomicalobservation.

Cost baseline This is the design baseline at the end of the conceptstudy, which was used to determine the cost of theSALT project, and on which the budget is based. It iscontained in the “SALT Report of Interim Project Team,April 1999”.

Flat field A type of calibration produced by a continuum lamp (e.g.quartz halogen) which is used to correct for sensitivityvariations, with as a function of field angle and/orwavelength, is a science instrument.

Growth Path This includes concepts that have not been fully explored,and do not form part of the deliverable. However, theseconcepts have to form part of the decision makingprocess in reaching the Technical or Cost Baselines.

Technical Baseline This is the design baseline that is required to fulfil therequirements of the SALT Observatory ScienceRequirements, Issue 7.1, and is the topic of thisSpecification. This baseline has not been costed, and thebudget implications will have to be ratified by the SALTBoard.



5

TABLE OF CONTENTS

1 Scope ..........................................................................................................81.1 Identification..................................................................................................................................81.2 System overview..........................................................................................................................81.3 Summary of calibration requirements.....................................................................................91.4 Instrument calibration operations............................................................................................91.4.1 System Efficiency: .....................................................................................................................91.4.2 Configuration time:....................................................................................................................101.4.3 Software and control efficiency:.............................................................................................101.4.4 Prior scheduling:.......................................................................................................................101.4.5 Interaction with the database:..................................................................................................101.4.6 Automation: ..............................................................................................................................10

2 Referenced documents ...........................................................................113 Customer Furnished Equipment and Responsibilities ........................124 Functional Requirements........................................................................134.1 Calibrations..................................................................................................................................134.2 Calculating Flat Fields ................................................................................................................134.2.1 Flat Fielding Process ................................................................................................................144.2.2 Flat Field Science Requirements ..............................................................................................144.3 Moving Pupil ................................................................................................................................154.4 Wavelength Calibrations ...........................................................................................................154.4.1 Wavelength Calibration science requirements.........................................................................15

5 PICS System Technical Requirements...................................................155.1 Interfaces.....................................................................................................................................155.1.1 External Interfaces...................................................................................................................175.1.2 Internal Interfaces ....................................................................................................................175.2 Schematic diagrams ..................................................................................................................175.2.1 PICS lamp box schematic .........................................................................................................185.2.2 PICS Illuminator schematic........................................................................................................185.3 Flat Field calibration lamp requirements...............................................................................195.3.1 Flat Field lamp type...................................................................................................................195.3.2 Lamp power supply and control ..............................................................................................195.3.3 Heat filtering .............................................................................................................................195.4 Wavelength Calibration Lamp requirements........................................................................195.4.1 Hollow cathode lamps..............................................................................................................195.4.2 Penray arc lamps .....................................................................................................................195.5 Lamp box requirements...........................................................................................................205.5.1 Light and airtight.......................................................................................................................205.5.2 Material and insulation..............................................................................................................205.5.3 Cooling......................................................................................................................................205.5.4 Power.......................................................................................................................................205.6 Filters............................................................................................................................................205.6.1 Colour balance filters ...............................................................................................................205.6.2 Neutral density filters ...............................................................................................................205.6.3 Filter wheel...............................................................................................................................205.7 Position for Calibration Illumination System ........................................................................205.8 Principle of the SOAR Concentrator Concept .......................................................................21



6

5.9 Optical requirements for the SALT illuminator....................................................................215.9.1 Diffuser ....................................................................................................................................225.9.2 Reflector optical tolerance .......................................................................................................225.9.3 Reflector coating......................................................................................................................225.10 Mechanical requirements of the SALT illuminator..............................................................225.11 Light guides.................................................................................................................................23

6 DICS System Technical Requirements...................................................236.1 Overview ......................................................................................................................................236.2 Calibration screen......................................................................................................................236.3 Illumination lamps......................................................................................................................236.4 Lamp positions...........................................................................................................................236.5 Colour filters ...............................................................................................................................23

7 PICS and DICS Operational Requirements ............................................247.1 Safety ............................................................................................................................................247.2 Physical Characteristics............................................................................................................247.2.1 Envelope...................................................................................................................................247.2.2 Mass.........................................................................................................................................247.2.3 Maximum & Minimum surface temperatures.............................................................................247.3 Component/module replacement...........................................................................................257.4 Environmental Requirements..................................................................................................257.4.1 Normal Operational Environment ..............................................................................................257.4.2 Marginal Operational Environment............................................................................................267.4.3 Survival Environment................................................................................................................267.5 Control..........................................................................................................................................277.5.1 Lamp control.............................................................................................................................277.5.2 Illuminator control......................................................................................................................277.5.3 Mirror motion control.................................................................................................................277.5.4 Filter selection ..........................................................................................................................277.5.5 Signal, control and data connections.......................................................................................277.5.6 Software and user interface ...................................................................................................277.5.7 Interlocks..................................................................................................................................287.6 Operation and Maintenance Requirements ..........................................................................287.6.1 Packaging, handling, storage ...................................................................................................287.6.2 Product Documentation ............................................................................................................287.6.3 Spares......................................................................................................................................287.6.4 Changing of lamps....................................................................................................................297.6.5 Illuminator and light guides........................................................................................................297.7 Availability ....................................................................................................................................297.7.1 Reliability...................................................................................................................................297.7.2 Maintainability ...........................................................................................................................297.7.3 Measures to achieve efficiency...............................................................................................29

8 Testing......................................................................................................308.1 Verification cross-reference matrix.......................................................................................308.2 Detailed Test Methods ..............................................................................................................30

9 Notes.........................................................................................................31

Appendix A: List of TBD’s and TBC’s



7

Modification History

Revision Changes Pages effectedA New Document All



8

1 Scope

1.1 Identification

This document specifies the requirements (the Technical Baseline) for the Southern African LargeTelescope (SALT) instrument calibration systems. The main system, which will form one of the PrimeFocus Payload (PFP) subsystems, is called the Payload Instrument Calibration System (PICS). Asecondary calibration system, involving an illuminated dome screen, is referred to as the Dome InstrumentCalibration System (DICS). This is an internal document that will be used to guide the calibration system’sdesign developments.

SALT is a reflecting optical astronomical telescope of the tilted Arecibo type, based on the design of theHET. The TCS forms the integrating node of the telescope and provides the Man-Machine Interface (MMI)to the SALT Operator (SO), the SALT Astronomer (SA) and the Principal Investigator (PI).

In general, the word “shall” is used to indicate mandatory requirements while descriptive statementsare used to provide non-mandatory information.

1.2 System overview The purpose of SALT is to collect light from astronomical objects, accurately focus it onto thetelescope focal plane from where it will proceed into an optical instrument while tracking the relativemovement of the target across the sky to maximise exposure time. The SALT system is comprised ofthe subsystems shown in Fig. 1 below:

Figure 1. SALT Subsystems

This specification will focus on the requirements for the SALT Instrument Calibration System (SICS),numbered 15?? in the breakdown of Fig. 1, consisting of the Payload Instrument Calibration System (PICS)and the Dome Instrument Calibration System (DICS) .

The SALT Payload Instrument Calibration System (PICS) comprises of a set of hardware mounted on thePrime Focus Payload (PFP), which is controlled through the Telescope Control System (TCS) either viaindividual instrument control computers or through the SALT Payload Computer (TBC3).

1000TelescopeSystem

1100Facility

1200TelescopeStructure

1300Dome

1400Primary Mirror

1500Tracker &Payload

3000ScienceInstruments

1700TCS

15??SICS



9

The PICS consists of a series of calibration lamps from which light is transferred, via light guides, to anillumination system. This illumination system can be controlled to move into the entrance of the SphericalAberration Corrector (SAC), allowing calibration observations to by undertaken. These can be doneduring the night, or alternatively scheduled for daylight hours immediately following a night’s observations. A dome calibration system (DICS), for flat fielding only, can also be employed during daylight hours ifnecessary.

The PICS consists of a fixed lamp box, housing all of the calibration lamps and associated optics, to whichthere will be coupled light guides. These light guides will then follow a routing down to the illuminationsystem, near the entrance to the SAC. The illumination system is spherical concentrator, with a diffuser,closely based on the design used by Gemini and SOAR. It can be moved into the beam under TCS orinstrument control and efficiently illuminates the M2 SAC mirror in a manner that will allow both flat fieldand arc calibrations to be undertaken.

The DICS comprises of a bank of several quartz-halogen lamps, mounted on either the top hex of theSALT structure, the dome catwalk or the dome itself. These lamps illuminate the inside of the domeopening, which is painted a matt white.

1.3 Summary of calibration requirementsThe calibration systems will provide flat-field (both PICS and DICS) and wavelength calibration (PICS only)CCD frames for all SALT first generation instruments (PFIS, SALTICAM and HRS) and potentially for futureinstruments. The first generation instruments have a variety of potential configurations, but have twocommon attributes: a science field of view of 8 arcmin diameter and a wavelength capability of 320-900nm (with some, albeit nominal, sensitivity down to 300 nm). The calibration systems should thereforesupport the calibration of science observations with these instruments, which cover both this spatial andspectral domain.

The required attributes of the calibration systems can be summarized as follows:1. Uniformity of calibration beam. The output beam profile needs to be as flat and as spatially and

temporally stable as possible.2. Constant illumination. The calibration lamps, particularly the flat field lamps, should have a near

constant flux output over time.3. Efficient. The calibration systems should be as efficient as possible to allow calibration observations

to be done in the shortest possible time.4. Filters. There should be provision for both neutral density (ND) and colour balance filters to allow the

spectral output of the calibration systems to be tuned to a degree. Such filters will be used to avoidsaturation or to enhance dynamic range.

5. Control. All calibration lamps will be mounted in a light and heat-proof housing and will be remotelycontrollable. The calibration illumination system for the SCS will be deployable and ready to begincalibration observations within a timescale of 30 sec.

1.4 Instrument calibration operationsInstrument calibration observations will be undertaken by either the SALT Astronomer (SA) or the SALTOperator. Some calibrations may be required immediately proceeding or following an observation of acelestial object (e.g. an arc calibration for wavelength determinations). Most other calibrations (e.g. flatfields) will be undertaken during the period immediately following a night’s observing. This could be in thelatter part of the night (e.g. if the weather forces an early cessation of observing), but will usually be inthe dawn twilight into the early morning period.

To avoid needless waste of precious observing time, the calibration observations should be able to beconfigured easily and undertaken in the minimum time. The latter implies that the calibration system shouldbe as efficient as possible, with the following requirements:

1.4.1 System Efficiency:The design should be ensure efficient light throughput throughout the whole optical train.



10

1.4.2 Configuration time:The time to configure the calibration system to the desired set up (i.e. choosing lamps, poweringup/down, inserting the illumination system) should be as short as possible.

1.4.3 Software and control efficiency:The control software should involve the minimum number of simple commands in order to execute acalibration observation and any latencies kept to a minimum.

1.4.4 Prior scheduling:Sequences of calibration observations shall be able to be scheduled in advance using the SchedulingTools available to the SA (or SO). Calibrations during the night will also be able to be scheduled atshort notice.

1.4.5 Interaction with the database:When defining a sequence of calibration observations, the observational database for the night willneed to be interrogated in order to provide the correct moving pupil baffle (MPB) tracks in order tomimic the pupil filling of the on-sky observation.

The calibration observations will be so named such that there will be a unique and well definedrelationship between the on-sky observation filenames and the calibration filenames. The calibrationfilenames will often be ‘shared’ amongst several on-sky observations.

1.4.6 Automation:The calibration observations scheduled after a night’s observing shall be able to be automaticallyundertaken in complete safety, with the following operations being controlled by the software:

1.4.6.1 Powering up/down of lamps1.4.6.2 Rotating selection mirror to desired lamp(s)1.4.6.3 Inserting the illumination system in front of the SAC1.4.6.4 Selecting the desired filters1.4.6.5 Ensuring calibration frames are not saturated by adjusting exposure times if necessary1.4.6.6 Sending relevant commands to the MPB in order to define its trajectory during a calibration

exposure



11

2 Referenced documents

SALT-1000AS0007 SALT System SpecificationSALT-1000AS0013 SALT Electrical Interface Control DossierSALT-1000AS0014 SALT Physical Interface Control DossierSALT-1000AS0040 SALT Operational RequirementSALT-1000BS0010 SALT Software StandardRamsay-Howat et al.1997

A dedicated calibration facility for the Gemini telescopes’, by S.Ramsay Howat, J.W. Harris & R.J. Bennett (SPIE 2871, 1171)

Ramsay Howat 2001 Private communicationIngerson 2000 ‘A proposal to build ISB/guide/calibration systems for SOAR’ by T.

Ingerson (http://www.ctio.noao.edu/soar/isb/isb_prop.html)Ingerson 2001 Private communications



12

3 Customer Furnished Equipment and Responsibilities

The Science Instruments and their computers are outside the scope of the Telescope project. Thesesystems and their MMI/interface software are “Customer Furnished Equipment”.

Details of the data and functional interfacing will be agreed and documented in the SALT InterfaceControl Dossier and each instrument’s own specification.

The data connection to the existing data network at Sutherland and the Internet, though not CFE, willrequire close liaison with the SAAO.



13

4 Functional Requirements

4.1 CalibrationsIt is a requirement of SALT that a suitable instrument calibration system(s) be provided for the scienceinstruments. Such calibrations include:

• flat-fielding the detector response:Flat field frames are used to correct for pixel sensitivity variations across the CCD (either intrinsicto the CCD or as a result of the instrument), as well as illumination non-uniformities. These areobtained by uniformly illuminating the CCD detector in the instrument.

• providing a wavelength calibrationShort exposures of an emission line source are used to determine the pixel-wavelength relationshipin a spectrum. This is achieved by illuminating the spectrograph entrance aperture (a slit, slitlet maskor fibre) with light from a hollow cathode discharge lamp, ideally in the same manner as a celestialobject (i.e. same optical path).

The calibration of SALT observations will form an important aspect of SALT operations and will be crucialin order to have science data of the highest quality. Both the SALT design, with its moving pupil, plus thequeue-scheduled observing operation complicates calibrations. The former requires a moving baffle inorder to duplicate the same amount of time varying pupil filling in the calibration observation as occurredin the on-sky observation.

Different observing programs conducted in a night using at least different instrument configurations, andpotentially even different instruments, leads to potentially many calibration observations having to beobtained. Particular operational modes increase the importance of standardized, automated calibrations,which can be carried out in the course of a night (e.g. arc calibrations) and in the twilight or daylighthours immediately following the observations (e.g. flat fields).

4.2 Calculating Flat FieldsOne aim of a flat field is to remove high frequency pixel-to-pixel sensitivity variations within a CCD.Likewise, a flat field is also used to remove lower frequency trends in instrument response, which areprimarily due to the optical effects of the instrument in question, rather than the CCD detector. This caninvolve the use of both calibration flat fields taken with the PICS or DICS, but normalized to a sky flat field.The latter is important when details of an astronomical source’s spectral behaviour are required. Similarly,the sky corrected flat fields are necessary to characterize the field-dependent illumination, whichdetermines the relative througput of multiple slits or fibres distributed at different field angles in the focalplane for MOS (Multiple Object Spectroscopy).

In essence a flat-field is a two-dimensional pixel sensitivity map, including large scale gradients andvignetting caused by dust particles on filters, CCD windows, etc. The signal to noise ratios (S/N) for low-contrast CCD images is limited by pixel non-uniformity over most of the dynamic range. Such non-uniformities can be intrinsic to the individual light-sensitive pixels (i.e. within the CCD substrate) or theymay occur as a result of other effects, like time-dependent dust accumulation on optical surfaces closeto the CCD (e.g. dewar windows, filters, etc) or the result of interference within the transparent Si layerof the CCD (i.e. fringing). Such sensitivity variations can therefore vary on a spatial scale of one to manytens of pixels.

The excellent linear response of CCDs permit the technique of ‘flat fielding’ out these variations using asimple algorithm which adjusts the sensitivity of each pixel to the same level by dividing the ‘object pixel’by the ‘flat field pixel’. The mathematical formula for this process is given as:

i

isi f

rc m=



14

Where:ms is the mean pixel value of a n ¥ n sub-array of the flat field CCD frameri is the value of the i th pixel in the raw CCD framefi is the value of the i th pixel in the flat field CCD frameci is the value of the i th pixel in the corrected CCD frame

The size of a typical sub-array used to determine the mean value depends on the spatial scale of lowfrequency variations and whether or not these variations are required to be removed. If only pixel-to-pixelchanges are to be removed, then n could be as small as 4 or 5.

4.2.1 Flat Fielding ProcessFlat field frames need to be of a high S/N ratio in order to be useful in correcting both for intrinsic pixel-to-pixel sensitivity variations within a CCD as well as the non-uniformities in the illumination of the CCD (i.e.from instrumental and optical effects). Suitable flat-field frames are usually obtained from co-adding many(typically 10-20) frames to produce a master frame. For dome flats (i.e. with the DCS), this is usuallystraightforward, since exposures are typically only a few seconds. Twilight, and in particular, night skyflats are more problematic because of the changing sky brightness in the case of the former, and the longexposure times needed in the case of the latter.

Flat field frames are needed for all the different filters, wavelength/grating positions and instrumental setup positions as used for the object frames. Ideally the flat-fields should illuminate the CCD in exactly thesame manner as in the case for the object exposures. This is to ensure that the light rays follow the samegeometrical paths to the detector. This is particularly important when correcting for CCD fringing, wherethe angular illumination of the CCD has a marked effect. The flat-field illumination of the instruments shouldbe over their entire FoV, which is at least 8 arcmin in diameter (although SALTICAM’s FOV is strictlyspeaking 10’ x 10’), and covering the spectral range 300-900 nm.

4.2.2 Flat Field Science RequirementsThe assumption when flat fielding a detector is that the illuminating flux is uniform and therefore eachindividual pixel receives the same flux as others. The calibration beam at the output of the PICS must besmoothly varying and the spatial intensity profile able to be well characterized and understood.

This will be achieved by a combination of the intrinsic uniformity of the beam as it exits the PICS andsubsequent calibration of the beam profile with reference to some external ‘flat’ source, like the sky. Limitsfor the flatness and stability of the beam shall be as follows (TBC1):

4.2.2.1 The calibration system should attempt to duplicate, at the detector, the same illuminationpattern (in terms of intensity and the ray paths) as a uniformly illuminated patch of sky.

4.2.2.2 Average flatness to <5% over the 8 arcmin FoV.

4.2.2.3 Flatness to <1% over scales <1 arcmin.

4.2.2.4 Output beam may have a monotonic roll-off in intensity of <10% of the maximum flux to theedges (to within 10% of the edge) of the FoV.

4.2.2.5 The flat-field beam shall have a temporally stable profile with a <1% variation of any point inthe profile from the mean value over a timescale < 12 hours).

4.2.2.6 The flat-field lamp shall have a constant intensity and spectral output of better than 2% afteran initial warm-up period of <20 minutes.

4.2.2.7 The flux of flat field lamps should be sufficiently high to achieve a S/N of >200 per detector



15

pixel (unbinned) in an exposure time of <5 sec, except for the near UV wavelengths wherethe requirement is relaxed to a S/N > 200 in < 30 sec for 300 nm < _ < 350 nm.

4.3 Moving PupilIn order to be as efficient as possible, many SALT calibrations will need to be undertaken during twilightor daylight hours immediately following the night’s observations. This is to both avoid wasting time doingcalibrations during useable observing time, and to ensure that calibration observations mimic the pupilillumination in the same manner as the on-sky observations.

The Moving Pupil Baffle (MPB) system, another component of the PFP, is crucial in making calibrationobservations, since it mimics the time varying entrance pupil to the instruments experienced during anobservation. This is necessary for the highest precision flat-fielding corrections (pixels illuminated in thesame manner) and for accurate wavelength calibration (line centroiding).

4.4 Wavelength CalibrationsA set of arc lamps will provide for wavelength calibration of spectrographs over the interval, at least from~300nm. PFIS is expected to have some, albeit greatly diminished, sensitivity from ~300-320 nm, (whichis of interest primarily for comet observations) and up to ~900nm for a variety of potential resolutions(R~100 to 100,000).

Several different lamps will be employed to give a sufficient number and density of unblended spectrallines over the entire wavelength range and for varying resolutions. For the fibre-fed HRS, the throughputof the Th-Ar/Th-Ne arc calibrations must be sufficiently high for efficient measurement of the faintestspectral lines.

4.4.1 Wavelength Calibration science requirementsThe calibration sources should illuminate the telescope focal plane in the same manner as theastronomical object(s), and with sufficient flux to obtain a high signal/noise ratio (S/N) calibration framein a short timescale for observing efficiency. For low-medium resolution spectroscopy (R < 10000) arclamp exposures of <20 sec should provide emission line spectra with line fluxes >1% of the maximum lineflux and with S/N >10. For higher resolutions (R > 30,000), mean emission line separations should be0.2nm, typical of Th-Ar arcs, and provide a S/N > 10 for exposures <20 sec.

Wavelength calibrations may be required during the night, in which case the exposure and setting uptimes for such calibrations must be short (< 30 sec in total).

5 PICS System Technical Requirements

This section defines the specific technical requirements or characteristics of the Payload InstrumentCalibration System (PICS).

5.1 Interfaces

The figure below shows the layout and positions of the SAC indicating the position of the PICSilluminator.



16

Figure 2. SAC layout and position of PICS illuminator

The figure below describes all the interfaces relevant to the PICS.

Position of exit pupil

Focal plane

Hole in M3:position forcalibrationilluminator

Non-Rotating PayloadStructure

Not Part of the PICS

INTERFACES E** : external

E1

PIC

S

Pay

load

Co

mp

ute

r

E2

Mo

vin

g P

up

ilB

affl

e

E5

SAC

E4

Fa

cil

ity E6

Inst

rum

ent

Co

mp

ute

r(s)

E3



17

5.1.1 External Interfaces

The PICS interfaces shall comply with the Physical, Electrical and External Interface Control Dossiers.Table 1 lists the relevant external interfaces as shown in Figure 3 above.

Table 1 External interfaces

5.1.2 Internal

Interfaces

The details shall be finalized during the design phase, but are mentioned in the concept describedbelow.

5.2 Schematic diagramsThe following diagrams show the proposed concept for the PICS layout, including the lamp box, light guideand the illumination system. Several different lamps (e.g. hollow cathode arc lamps, quartz halogenlamps) will be mounted in a circular box structure. Each lamp housing will contain a simple focus bi-convex lens, which relays the light from the cathode/filament to a liquid light guide, via a rotating double-sided flat mirror. The latter can be positioned to select one of several lamps to feed the two light guides,which in turn are relayed to the illumination system. The two light guides have different transmissionproperties, one optimised for shorter wavelengths (300-600 nm; referred to as ‘blue’) and one for longerwavelengths (400-900 nm; ‘red’). The mirror should be able to direct light from any of the lamps to anyof the fibres, and even simultaneously feed the two fibres at once.

No. Subsystem 1 Subsystem 2 Type Direction Interface DescriptionPhysical Both Mounting

Physical Structure Connections for DataPhysical Structure Connections for AirPhysical Structure Supply and return connection for

cooling

E1 Non-RotatingPayloadStructure

PICS

Physical Structure Light guide conduitsE2 Payload

ComputerPICS Data Both Communication

E3 InstrumentComputer(s)

PICS Data Both Communication and motioncontrol

E4 SAC PICS Physical Both Mounting of PICS illuminator toinsertion system. Should locateas close to SAC entrance hole aspossible (<5mm), but withoutchance of collision

E5 Moving PupilBaffle (MPB)

PICS Data MPB Trajectory information and timesfor baffle motion

E6 Facility PICS Electric PICS PowerCooling PICS Glycol – for cooling lamp box

Figure 3. PICS interfaces



18

5.2.1 PICS lamp box schematic

0 5025 150

Figure 4. Schematic layout of the PICS lamp box

Figure 4 is a schematic of the SCS lamp box, mounted on the PFP at some convenient location. Lightfrom a variety of wavelength calibration arc lamps, plus a Quartz Tungsten Halogen (QTH) flat fieldlamp can be fed into either of the liquid light guides.

5.2.2 PICS Illuminator schematicThe illuminator (discussed later in 5.8 and 5.9) is likely to consist of a hemispherical reflector, with adiffuser at the centre of curvature. Light will be fed in by the light guides, possibly with small lensescoupled, to efficiently illuminate the diffuser, which in turn scatters light back. Some of this light exits thesystem and enters the SAC, while the rest is reflected by the spherical reflector back on to the diffuser,and so on. Figure 2 shows the GEMINI ‘concentrator’ calibration system for reference. The difference forthe SALT system will be the replacement of lamp fixtures with light guide connections.

QTH flat fieldlamp

Blue liquidlight guide

Red liquidlight guide

HgCdZn arclampCuAr arc

lamp

ThAr arclamp

HeNe arclamp

Xe,Deuteriumor Penraylamps

6-position doublesided stearing mirror



19

Figure 5. The GEMINI illumination / concentrator system on which the PICS will be based.

5.3 Flat Field calibration lamp requirementsFlat field lamps should provide a continuous spectrum across the entire wavelength domain of thefirst generation instruments, i.e. 320-900 nm, with a (greatly reduced) capability to 300 nm.

5.3.1 Flat Field lamp typeFor efficiency, particularly in the blue, a bright Quartz Tungsten Halogen (QTH) incandescent lamp shouldbe used for flat fielding. Examples include the General Electric 787 (6.0V, 1.67A), although accordingto the HET, Gilway Technical Lamp make an equivalent but better bulb and also provide bulb sockets.Gilway’s lamps are unique in that the quartz does not have any additives which inhibit the UV light.

5.3.2 Lamp power supply and controlFor QTH power supplies probably the best available is the HP E3634A or E3633A. These have extremelyconstant current mode, are fully programmable, and remotely controllable via either RS232 or HP-IB (IEEE-488). They can also be rack mounted side by side in a 5 _” rack space. HET bought their’s with aneducational discount at ~$950 each (2001 prices). If SALT decides on these using RS232 control, thenHET can offer tips on some of the subtleties of their operation.

5.3.3 Heat filteringThere shall be consideration of the issue of heat generation inside the lamp box, particulary from the QTHlamp, which could have a power of ~100W. Some IR filter may need to be employed to avoid undulyheating the liquid light guide.

5.4 Wavelength Calibration Lamp requirementsThese lamps will be often be required to do wavelength calibrations before and an observation, usually(but not always) without using the moving baffle to simulate a track. On other occasions arcs may beobtained during twilight/daytime, in which case the moving baffle will be used in conjunction with thecalibration observations. Because of heat considerations, the lamps and any power supplies mountedwith, or near, them, should be housed in an insulated box with glycol plumbing installed to remove heat.

5.4.1 Hollow cathode lampsThe following selection of hollow cathode arc lamps will likely be required and like the current lamps inuse at SAAO will probably be sourced from Cathodeon in the UK, or Varian. We propose using thefollowing lamps:

a.) Fe-Ar or Cu-Ar lamp good primarily in the blue at low/medium Rb.) Ne good primarily in the red at low/medium Rc.) HgCdZn provides lines over wide range and suitable for low Rd.) Th-Ar or Th-Ne good for high Re.) Xe or Deuterium lamp for very high R.

The exact choice of lamps, and the number to be accommodated in the lamp box, will be determinedfollowing discussions with the instrument PIs (TBD1). However, since they all have essentially identicaloperational requirements (voltage, current, base sockets), dimensions, they will be fully interchangeable.

5.4.2 Penray arc lampsBecause of the relatively low flux of hollow cathode arc lamps, it will be preferable in certaincircumstances (e.g. night time calibrations) to use high intensity “Penray” lamps. These lamps are verybright and in most cases will give a good calibration exposure in a seconds. The lamps used will likely beHe, Ar, Ne and Xe Oriel pencil lamps (corresponding to Oriel part numbers 650202, 6030, 6032 and



20

6033).

5.5 Lamp box requirementsThe lamp box housing the calibration sources (e.g. QTH, arc lamps) shall conform to the followingrequirements:5.5.1 Light and airtightThe box shall be completely light tight and as air tight as practical. Light should only exit the boxthrough the light guides.5.5.2 Material and insulationThe box should be made of a lightweight material, and should be insulated.5.5.3 CoolingThe lamp box should be plumbed into the system glycol lines and should have some sort of fan and heatexchanger system to keep the air temperature in the box at close to the ambient. The lamp box should beremoveable such that the glycol line connectors avoid leaking on disconnection.5.5.4 PowerLamp power supplies will located in one of the igloos of the PFP, and he power routed up to the lampbox.

5.6 Filters5.6.1 Colour balance filtersBecause of the gradient in the flux of the QTH flat field lamps, where the flux in the red part of thespectrum (~700 nm) can be ~30 times that of the blue (~350 nm), it will be necessary to employ a filterto reduce this effect in order that there is not a severe mismatch in the signal to noise over the spectrum,or to avoid saturation in the red. Colour balance filters will be employed for this purpose, for exampleSchott BG series (e.g. BG12, BG34, BG24a, BG38, BG39), perhaps in combination with a UG5 filter, forthe most extreme blue observations. The exact choice of these colour balance filters is TBD2.

As in the case of flat field lamps, some filters will be required for some of the lamps in order to select linesfrom the relevant spectral orders, to modify spectral line intensities to avoid saturation. Colour filtersachieve wavelength dependent while neutral density (ND) filters can be used to control the overallintensity level of calibration and flat field exposures.

5.6.2 Neutral density filtersA range of ND filters shall be provided with optical densities (D) of 0.5, 1.0, 1.6, 2.0, 2.6 and 3.0,corresponding to transmissions (10-D) from 0.31 to 10-3.

5.6.3 Filter wheelThe filters (perhaps 9 in total; TBC2) shall be mounted in a filter wheel, or stage, allowing selectedfilters to be placed in the beam. The exact position of this wheel is TBD3, but could be in any of thefollowing positions:1. inside the lamp selector box, before light enters the light guide2. near the exit of the light guide3. at the exit of the illuminator

The preferred position is 1., since there are potential space and weight limitations with the otheroptions.

5.7 Position for Calibration Illumination SystemThe positioning of the SALT calibration illumination system (sometimes referred to as the “calibrationscreen”) was discussed in the discussion document SALT Calibration Issues and CCD Requirements(9 April 2002). For SALT, the three logical place to put a screen or illumination system are:

1. the exit pupil position2. at the entrance to the SAC3. on the dome



21

The latter has been adopted for the Dome Instrument Calibration System (DICS), to be discussed in section6. For the first two options, the relative merits of each are discussed in the above mentioned document.However, because of the limitation of space at the exit pupil position, due to various other PFP subsystem(e.g. the moving baffle, acquisition camera and fold mirror, ADC), it has been decided that option 2 willbe chosen for the position of illumination system for the PICS.

The advantages of this system include the potentially greater efficiency of illumination plus the desire tohave the calibration frames undergo the same degree of vignetting by the SAC as the on-skyobservations. In order to improve the efficiency, we have opted to employ a ‘concentrator’ of the typeused in both the calibration systems for Gemini (Ramsay-Howat 1997,2001) and SOAR (Ingerson 2000),and the basic design is shown in Figure 2.

5.8 Principle of the SOAR Concentrator ConceptThe following is a description of the calibration system devised for SOAR, based closely on the Geminisystem, and referred to as a ‘concentrator’. The next sections will discuss the adaptation of this conceptto SALT in terms of what we will call the calibration illumination system (equivalent in previous parlanceto the ‘calibration screen’).

Light from calibration lamps will be fed into liquid light guides, which in turn deliver the light to theconcentrator. The concentrator integrates the light in a reflecting hemisphere, superficially resembling anintegrating sphere, but significantly more efficient. The concentrator then directs the light efficiently intoa beam of a fixed f/ratio, defined by the parameters of the concentrator, but matched to fill the first opticof the SAC, namely M2.

In the Gemini/SOAR system, light from calibration lamps is directed, almost focused, via a lens, onto asmall (plane?) mirror on the inside of the hemispherical reflecting surface of the concentrator. From themirror, the diverging light is reflected onto a diffusing screen at the centre of the concentrator. Lightscattered from the diffuser creates a uniformly illuminated beam at the exit hole of the concentrator,which acts as a field stop. The area around the diffuser is of low reflectivity to reduce scattered light.

The f/ ratio of the exit beam is the ratio of the distance between the diffuser and the exit hole to thediameter of the hole. The amount of beam spread is determined from the size of the diffuser. A field lensat the exit hole is used to move the pupil to the desired location, in the case of SOAR, to an intermediatepupil stop ~110mm in front of this lens. The beam is then folded by an off-axis ellipsoidal mirror, whichconverts the beam to the telescope’s f/16 and delivers it to the focal plane. The output beam can thus bemade to simulate the telescope reasonably well.

Light leaving the concentrator through the exit hole is intercepted by the lens and then the mirror, andfocused on the field. Light missing the exit hole is reflected by the spherical polished surface of theconcentrator (actually a prolate spheroid is the correct shape for this), which directs it back to thediffuser, where it scattered a second time. Again, some fraction of this scattered light will leave throughthe exit hole, while some is again intercepted by the spherical mirror, and so on. Provided that the diffuserand polished sphere are well matched (in terms of the dimensions, radius of curvature, etc), all the lightleaving the concentrator will have the same beam properties in terms of relative flux density, beam profileshape, f/ratio, etc. This reflective version of an integrating sphere is far more efficient than the latter,since the time averaged beam is much brighter, and the light can concentrated into a useful beam of therequired f/ratio, rather than spreading unnecessarily over a full 2p steradians.

The Gemini/SOAR concentrator has an efficiency at least 10 times that of a conventional integratingsphere, being more efficient for concentrated light sources like QTH lamps.

5.9 Optical requirements for the SALT illuminatorUnlike the Gemini and SOAR calibration systems, it is not possible to create a virtual pupil at the samedistance from the SALT focal plane as the SALT exit pupil. This is because of the aforementioned lackof space in the payload near the exit pupil and the choice of using a calibration system outside of the



22

SAC, where the latter’s optics relay the calibration light to the focal plane.

The best that we can hope for is to attempt to simulate the ray bundles from an extended (8-10 arcmindiameter) patch of uniform sky, passing through the MI caustic (i.e. before filling M2). To this end thereare several free parameters to choose in designing the concentrator system, which are TBD4 followingsome ray tracing analysis.

1. the diameter of the spherical reflector2. the diameter of the concentrator exit hole3. the optical parameters of any field lens (diameter, focal length)4. the size of the diffuser5. the shape of the diffuser (e.g. flat, concave)

5.9.1 DiffuserThe diffuser must be chosen such that light falling on it is scattered uniformly and that it has no significantstructure which could be imparted on the flat field CCD frames. Because the diffuser is positioned closeto the paraxial focus of M1, it is possible for any such structure or defects (e.g. scratches, spots, dust)to be partially focused onto the detector, resulting in a compromised ability to remove high frequencypixel-to-pixel variations through the usual flat fielding process.

Although ground glass screens are typically used as diffusers, it is likely that the sub-structure of thesewould prove problematical. A better choice is probably an opalescent material, or a matte white coatinglike Labsphere’s SPECTRALON or DURAFLECT-II. Depending upon how the light guides interface with theilluminator will have some bearing on the type of diffuser chosen, i.e. reflective or transmissive, thebaseline being the former.

In addition, the use of some beam shaping (e.g. using holographic diffusers) may be desirable in achievingthis goal, and also boosting the overall efficiency.

Finally, it has been suggested (e.g. by Phillip MacQueen) that one way to remove any structure is torandomly ‘jiggle’ the diffuser in order to blur, or smooth out, any structure present on the diffuser (e.g.dust or defects). However, we note that several other calibration systems in use (e.g. the HYDRAsystem at CTIO) involves imaging a diffuser directly onto a focal plane, and these seem to work quitesuccessfully, although dust can be a problem. For the PICS, provided due care is taken to seal theilluminator, then these problems may be reduced.

5.9.2 Reflector optical toleranceThe tolerance required for the reflector is not expected to be particularly high, since it is not an imagingsystem as such, but this will be determined from the ray tracing or optical analysis.

5.9.3 Reflector coatingThe choice of reflector coating is TBD5, but is likely to be aluminium (Al) so as to achieve efficiency in theblue (i.e. down to 320 nm). Depending on the material choice for the reflector, it may be sufficient to applya conventional vacuum Al coating. If the reflector is made from Al (e.g. diamond turned), then some post-polishing may then be required to ensure high reflectivity, depending on the surface finish.

5.10 Mechanical requirements of the SALT illuminatorThe illuminator is to be positioned at the bottom of the SAC on a mechanical arm or stage which willallow it to move to one of two positions:1. The ‘in’ position: where the illuminator is positioned on-axis, close to the SAC entrance. It is important

that the illuminator position is repeatable, absolutely, to 15 microns accuracy with respect to the SAC.2. The ‘out’ position: where the illuminator is positioned at some convenient location where it does not

obstruct and light entering the SAC or hinder its full range of motion. Accuracy of locating to the outposition is not crucial.



23

The illumination system should be so positioned that the light guides conveying light from the lamp box arefree to move without hindrance or the possibility that they could become snagged on some part of theSAC, PFP or telescope.

The illumination system should be so designed to avoid dust entering the illuminator, since the diffuserneeds to be kept as dust free as possible. A sealed unit (e.g. with the field lens mounted to the exit holewith an o-ring) is to be preferred.

5.11 Light guidesIt is proposed that liquid light guides be employed in the SCS to convey light from the lamps to theillumination system. The choice of liquid light guides is dictated by the requirement of good throughput inthe blue, where normal fused silica fibres have a large degree of attenuation. Such light guides have beenused to good effect with the calibration system for the HYDRA spectrograph on the CTIO 4-m Blancotelescope.

It is proposed to use liquid light guides from Lumatec, which provide good throughput into the UV, andpeak transmission of ~80% for a 2-m length. The series 300 has >60% transmission in the region 320-600nm, which is ideal for observations in the blue/UV region. The series 380 light guide has >60%transmission from 350-750 nm, dropping to ~40% at 900nm, which would suite observations in the visibleand near IR.

Two liquid light guides could be employed (as proposed in section 5.1), one fed predominantly bywavelength calibration lamps mostly used in the blue (e.g. Cu-Ar, HgCdZn), the other by predominantlylamps for high resolution and/or red spectroscopy (e.g. Th-Ar, HeNe). Optimal placement of the QTH lampwould be at the 90° position to the two fibres, allowing both to be injected if necessary. Diameters of 5mmshould be used, which will allow for a minimum bend radius of 6cm, which should easily be met providedthe light guide routing is carefully chosen.

6 DICS System Technical Requirements6.1 OverviewThe Dome Instrument Calibration System (DICS) is a flat field calibration system consisting of a whitescreen on the dome illuminated by a bank of bright quartz halogen lamps. It will serve as an inexpensivebackup for the main calibration system, PICS, and will be useful for removing low frequency mismatchesin the response of flat field calibration. It will only be operated during daylight or bad weather, when thedome is closed.

6.2 Calibration screenThe calibration screen for DICS is simply the inner surface of the dome door opening, which is painteda matte white (already completed). At some future time (e.g. during commissioning) it maybe deemedadvisable to repaint the door with a proprietary material specially made for such screens. This will dependon the initial flat field results and the predicted utility of the DICS.

6.3 Illumination lampsThese will be banks of bright Quartz-Halogen spot lights, positioned either on the top hex of the SALTtruss, and/or the dome catwalk, or the dome itself. They will have searchlight type reflectors in order toefficiently illuminate a ~12 meter diameter circular spot on the inside of the dome door, immediately in frontof SALT.

6.4 Lamp positionsThe lamps should be so positioned that the illumination pattern is as uniform as possible, and that noshadowing occurs on the screen.

6.5 Colour filters



24

Depending on the spectral shape of the lamps, it may be required that colour balance filters be placedin front of some of the lamps in order to boost the amount of blue light relative to the red light.

7 PICS and DICS Operational Requirements7.1 SafetyThe following should be read in conjunction with the SALT Safety Analysis, SALT-1000AA0030,listed in section 2.

All single point failures that can lead to loss of life, serious injury to personnel or damage to equipmentshall be identified and the design modified to prevent such failures. In no case shall it be possible for anycomponent of the instrument to detach and drop from the Payload.

Motor overload protection, fusing and sensing shall be implemented and monitored by the control systemto ensure that failure mode criteria are met. Where tools must be used on-telescope for servicing andmaintenance, they shall be secured by lanyards to the servicer’s tool belt or man lift.

All fasteners, cover panels and other components which can be accessed while the tracker is ontelescope shall be captivated by the use of _ turn captured fasteners, wire loop, bails, threads or someother similar means to prevent accidental injury to personnel below as well as damage to primary mirror.

No lock washers shall be used for on-telescope accessible fasteners, chemical locking compounds oraircraft-type locking nuts shall be used instead.

A safety analysis and design shall be presented and implemented to satisfy all safety requirements.

Software development process to be commensurate with the safety implication of software failure(see SALT1000BS0010)

7.2 Physical Characteristics

7.2.1 EnvelopeNo part of the PICS shall protrude beyond a radius of 1.5 m from the optical axis of the telescope.Likewise, no part of the DICS shall protrude inside the beam of the telescope.

7.2.2 MassThe total PICS mass on the payload shall be less than 25 kg (TBD6).

7.2.3 Maximum & Minimum surface temperaturesAll the PICS related electronics and lamps shall be located in the instrument envelope on the payload ina cooled enclosure. Since DICS will only be used during daylight hours, the heat emission requirementscan be relaxed for this.

For PICS, the issue of heat generation is an issue to be addressed. Any gradient in the air temperaturewithin the optical path will have a negative influence on the image quality produced by SALT. In order tominimise this effect, the following constraints are imposed.

7.2.3.1 Objects in the optical pathAll items of equipment that are within 1m of the telescope optical path or within a vertical cylinder definedas a vertical extension of the pier to the highest point of the top hex, shall comply with the following:

(a) No item exposed to the ambient air, regardless of its size, shall have a surface temperature ofmore than 8ºC above ambient.(b) No item having forced-air cooling shall blow the exhausted air into the ambient air.



25

(c) No item exposed to the ambient air, regardless of its size, shall have a surface temperaturecooler than 2ºC below ambient to prevent condensation on surfaces.(d) No item shall blow exhausted cool air into the ambient air.(e) Items with a surface temperature of more than 2ºC above ambient shall have a Thermal Factor(TF) of less than 0.6 m2C, where TF is defined as follows:

TF = A__T

Where A = Exposed surface area of the item in m2

_T = Temperature difference between the items exposed surface and the ambient air temperature inºC

NOTE: In practice, these constraints mean that many items may require cooling jackets or cooledenclosures. As an example, an item measuring 0.4x0.4x0.4m emitting more than about 4W of heatcontinuously, will need to be insulated and cooled otherwise its surface temperature will go above theallowed limit.

7.2.3.2 Objects outside the optical pathAll items of equipment that are within 1 m of the telescope optical path, but not included in 7.2.3.1 shallcomply with the following:No item exposed to the ambient air, regardless of its size, shall have a surface temperature of morethan 8ºC above ambient.No item having forced-air cooling shall blow the exhausted air into the ambient air.No item exposed to the ambient air, regardless of its size, shall have a surface temperature coolerthan 3ºC below ambient.No item shall blow exhausted cool air into the ambient air

Items with a surface temperature of more than 3ºC above ambient shall have a Thermal Factor (TF) ofless than 2 m2C (with TF defined in 7.2.3.1).

NOTE: In practice, these constraints mean that many items may require cooling jackets or cooledenclosures. As an example, an item measuring 0.4x0.4x0.4m emitting more than about 6.5W of heatcontinuously, will need to be insulated and cooled otherwise it’s surface temperature will go above theallowed limit.

7.3 Component/module replacementAll major components that might need removal (e.g. lamp box, illuminator, but excluding smallsubcomponents like lamps), must be designed so that they can be handled easily by maintenancepersonnel. Any special lifting or handling fixtures for modules by their nature or orientation require suchfixtures for safe lifting and positioning. Any individual module of the camera shall weigh less than 15 kgso that a single person can manhandle individual modules without strain.

The size of the component must be such that the item can be removed through access relevant accessdoor/panels on the PFP.

7.4 Environmental Requirements

7.4.1 Normal Operational Environment

SALT shall meet all the requirements specified in this document when operated in the night-timeoutside ambient condition defined in Table 2 below:



26

Table 2 Normal Operational Environment

Parameter Value NotesMinimum Temperature 0ºCMaximum Temperature 20ºCMaximum nightly temperaturerange

8ºC

Maximum rate environmentcooling

-1.5ºC/h

Maximum rate of environmentwarming

+0.5ºC/h Estimated value

Minimum Humidity 5%Maximum Humidity 97% Non-condensingMaximum wind velocity(outside)

16.8 m/s Gusts up to 22 m/s

Maximum wind velocity (atdome opening)

6m/s

Site altitude 1798mSolar radiation 0 W/m2 Twilight to dawn

7.4.2 Marginal Operational Environment

The degradation of system performance as a result of the ambient environment specified in Table 3below, shall not exceed 10% of the nominal values in paragraph 7.4.1.

Table 3 Marginal Operational Environment

Parameter Value NotesMinimum Temperature -10ºCMaximum Temperature 25ºCMaximum rate environmentcooling

-2.0ºC/h

Maximum rate of environmentwarming

+1ºC/h Estimated value

Minimum Humidity 5%Maximum Humidity 97% Non-condensingMaximum wind velocity(outside)

21 m/s Gusts up to 25 m/s

Maximum wind velocity (inside) 8m/sSolar radiation 0 W/m2 Twilight to dawn

7.4.3 Survival Environment

SALT shall survive when exposed to the day or night ambient environment specified in Table 4 below.Note that the dome and louvres will be closed under these conditions and therefore the tracker does nothave to be designed for this wind loading, but all tracker subsystems must be able to survive thetemperature profile.



27

Table 4 SALT Survival Operating Environment

Parameter Value NotesMinimum Temperature -20ºC**Maximum Temperature 45ºC**Maximum Humidity 100% Occasional exposure to

condensing conditionsMaximum wind velocity(outside)

61 m/s**

Rain Note 1Snow Note 1Hail Note 1Icing Present Low temperatures after

condensation or rain arecommon.

Solar Radiation Note 1Other Note 1NOTES:Environmental conditions not specified shall be obtained from the appropriatebuilding/civil standards suitable for Sutherland.**: Use the worst case of these figures and those specified in the appropriatebuilding/civil standards.

7.5 ControlBoth the PICS and DICS should be controllable through software either residing on, or part of, theinstrument control computer(s), and the SALT TCS, through the Payload Computer (TBC3). Thefollowing functions need to be controlled:

7.5.1 Lamp controlAny subset of lamps should be able to be turned on or off remotely. An indication of lamp current shallbe provided. On the issuing of a lamp selection command, the pick-off mirror will automatically rotateto the position required to inject the light into the light guide.

7.5.2 Illuminator controlThere shall be an illumination motion control system, which moves the concentrator into position nearthe hole in M3. Lights will indicate the status of this system (i.e. in-beam for calibrations, out-of-beam,moving or failure).

7.5.3 Mirror motion controlThe rotating pickoff mirror will be mounted on an encoded motor (e.g. stepper motor), under controlfrom the PICS software.

7.5.4 Filter selectionFilters selected for calibrations will be inserted into the beam when the relevant control command inthe PICS software is issued.

7.5.5 Signal, control and data connectionsThe PICS and DISC shall receive and provide all signal, control and data paths which are interfacedthrough the Payload Computer.

7.5.6 Software and user interfaceThe SA and SO will interact with the calibration system either through a stand alone GUI on the PayloadComputer, or through a calibration GUI on the Instrument Computer(s). Exact details of this are still TBC3.



28

Some information from the SICS needs also to be passed to the Event Logger and the Science Database.Examples include the status of lamps (on/of), illuminator (in/out), mirror and filter positions, details ofcalibration observations completed (e.g. calibration configuration, exposure times).

The SICS software should be efficient and minimal, to avoid the necessity of issuing too many commands.This shall particularly be the case for calibrations obtained during the night. Calibration set up scripts willusually be produced, containing the specific details of the calibration observations. In addition, there shallbe a manual mode with a number of ‘configure’ commands in a drop-down menu, followed by exposuretime requirements. For (usually daytime) calibrations involving the use of the moving baffle, requiredinformation for the moving pupil baffle (MPB) will be supplied in the calibration set up file, derived from theScience Database.

In the automatic mode (daylight hours), the calibration software should be able to handle switching lampson and off, moving filter and controlling the MPB with no user intervention. At the completion of thiscalibration phase, the calibration CCD frames will be automatically stored on the database, withinformation in their headers indicating to which specific observations they pertain (i.e. where the tracktrajectories were derived). The software needs to alert the Event Logger when it is in this automaticmode, and must conform to the specified SALT Safety Standard.

7.5.7 InterlocksSoftware and hardware interlocks will be required to avoid the following situations:

7.5.7.1 Either the PICS or DICS being operated when on-sky observations are being conducted.7.5.7.2 Avoid leaving lamps turned on after completing calibration observations.7.5.7.3 Avoid leaving the PICS illuminator obstructing the SAC after a calibration.7.5.7.4 Avoid potentially over-illuminating any exposure meters inside instruments when the latter

are running (i.e. either automatically turnoff the meter or prevent the lamp turning on.

7.6 Operation and Maintenance Requirements

7.6.1 Packaging, handling, storage

Packaging, handling and storage requirements will be determined for each individual type of component,taking into account the specific requirements of the component, the method of shipping and interimstorage locations. Storage at SALT will be in the SALT Store Room, in dry, air-conditioned conditions. Containers shall be sufficient for one return shipping only, unless otherwise specified.

7.6.2 Product Documentation

The SICS instrument shall include any relevant product spec sheets, operating manuals, maintenancemanuals, calibration procedures and component level documentation.

Full size copies of as built component specifications, drawings and CAD files.

Acceptance test documentation.

Build History document.

All documentation shall be in English.

7.6.3 SparesSufficient spares of crucial items for the PICS and DICS should be carried, which includes:



29

7.6.3.1 Arc lamps (excluding socket)7.6.3.2 QTH bulbs

7.6.4 Changing of lampsThe calibration lamps will be mounted inside a lamp box mounted on the PFP. This box needs to be easilyaccessible to allow for the removal and replacement of lamps, during the night, if necessary.

It is expected that the lamps (QTH and arcs) in both the PICS and DICS will need to be changed fromtime to time. The exact frequency of this operation is not certain, but in most cases will be seldom.Since all of the hollow cathode arc lamp housings and sockets are common, it should be relativelystraightforward to replace a broken lamp. For QTH bulbs, however, may have filaments which vary intheir positions from bulb to bulb, possibly affecting the illumination uniformity and beam shape.Although it shall be possible to keep the pins of the bulbs in the same place on the PICS base socket, itis not possible to guarantee repeatable positioning of the filaments. This implies that flat field uniformitymay need to be re-calibrated following the replacement of a flat field QTH bulb.

7.6.5 Illuminator and light guidesThese components should be positioned to allow any required maintenance during daylight hourswithout requiring a major dismantling of the PFP to access them.

7.7 Availability

7.7.1 Reliability

• Unscheduled downtime during telescope night time operation shall not exceed1hr/year.

• Expected use of the PICS is ~100 translations of the illuminator system into thebeam per night (maximum range) over a lifetime of 30 years. Lamps will need to beturned on/off with a similar frequency.

• Lamps and power supplies should be so chosen, where possible, on the basis oftheir expected lifetimes.

• Scheduled maintenance of the SICS (PICS and DICS) will include lamp changingand power supply maintenance, which shall be carried out on the PFP in situ.Therefore access shall be made as easy as possible to both the lamp box andilluminator.

7.7.2 Maintainability

• Scheduled maintenance shall be performed during daytime hours and shall notexceed 1hour per month.

• Spares shall be provided for components critical to SICS operations (both PICS andDICS) whose failure will lead to down time of more than the specification in 7.6.1.

7.7.3 Measures to achieve efficiency

Subsystems shall be organized into modules for ease of mounting/dismounting and servicing.

COTS equipment will be used as far as possible to reduce spares holding requirements. A float levelof standard spares (bolts, nuts, wires, oils, grease) will be kept in the SALT Store.

As far as possible local support for all subsystems/components is required

Special tools and equipment required for system operation and maintenance shall be kept to a



30

minimum, and will be provided with each subsystem.

All normal maintenance actions will be able to be completed within one working day, unless otherwisespecified. Where maintenance actions take more than a day and happen regularly (e.g. primary mirrorcoating), enough spares will be held to ensure that the operation of the telescope system is notaffected.

Two standard (metric) tool sets will be available, one in the SALT workshop and one in the telescopechamber. Special tools shall be kept to a minimum.

8 Testing8.1 Verification cross-reference matrix

This section will indicate how system compliance to the requirements in the preceding sections will beproven. Various types of verification (Testing, Analysis, Demonstration and Inspection) will be used atvarious levels of system integration (System, Subsystem and component) as appropriate.

Para. Requirement TestLevel

[Note 1]

TestMethod[Note 2]

Test DetailRef.

[Note 3]

Note 1: “Test Level” may be “system” (S), “subsystem” (SS) or component (C) level,depending on the particular requirement .

Note 2: The "Test Method" may be any one of the following:• Review (R) - the design is reviewed and it is obvious to all whether or not the item

complies (e.g. whether or not the system has a particular mode).• Inspection (I) - the completed item is inspected and compliance can be easily

observed. This is normally used for physical characteristics such as colour,dimensions and mass.

• Testing (T) - this entails a technical effort whereby the system is stimulated in acertain fashion and its response compared to the required response.

• Analysis (A) – compliance of the design to the requirement is proved by mathematicalanalysis.

Note 3: Where it is considered important, reference to the detail of the test method describedin section 8.2 is be provided in the "Details" column of the table

Table 11: Verification Cross-Reference Matrix

8.2 Detailed Test Methods

Where applicable, provide details of particular tests that will be done. (Especially for potentiallycontentious requirements or critical performance criteria.)



31

9 NotesIssue A of this document is for preliminary review and is not suitable for final use.



32

Appendix D: List of TBD’s and TBC’s

The following issues are addressed in this document but require definition (TBD’s – To Be Determined) orconfirmation (TBC’s – To Be Confirmed):

TBC/TBDnumber

Topic Comments

TBC1 Flatness of flat fields Await modelling resultsTBC2 Number of filters Consult instrument PIsTBC3 Software control through TCS Await discussions with GS

TBD1 Choice of arc lamps Discuss with PIs, usersTBD2 Choice of colour filters Await further analysisTBD3 Position of filters in PICS Await design detailsTBD4 Parameters of illuminator/concentrator Await optical modelling resultsTBD5 Reflective coating for illuminator Await mechanical and optical

design progressTBD6 PICS mass Await preliminary mechanical

design

Download - TITLE : SALT Instrument Calibration System · TITLE : SALT Instrument Calibration System ... lamps; calibration screen; illumination PREPARED BY : David Buckley SALT Project Scientist

Top Related